The preliminary evaluation of the Focus electric cars from 2012 and 2013 model years was opened after 11 consumers alleged their vehicle shut down while driving, according to documents filed with the National Highway Traffic Safety Administration. An estimated 1,000 of the plug-in electric cars, launched last year, are included in the probe.

Half of the incidents occurred at more than 30 mph, and all of the complaints came within the past five months, according to the NHTSA. No accidents or injuries were reported.

A Ford spokeswoman said the U.S. automaker is cooperating with the NHTSA’s probe.

A preliminary evaluation is the first step in a process that could lead to a recall, should regulators determine that a safety issue needs to be addressed by a manufacturer.